1. Field of the Invention
The invention relates to a process for deaerating liquids, especially for deaerating aqueous suspensions as obtained, for example, in textile treatment or pulp and paper production.
2. Description of the Related Art
In many liquid systems, especially aqueous systems, which contain surface-active compounds as desired or else as undesired constituents, problems can occur with entrapped air bubbles when these systems are brought into more or less intensive contact with gaseous substances, for example in the sparging of waste waters, in the intensive stirring of liquids, and in distillation, washing or dyeing operations. Especially liquids containing particles in fine distribution which can attract air bubbles, for example fibers, tend to entrap air.
In pulp production, the entrapped air prevents, for example, rapid drainage of the water and thus lowers quality and productivity.
Surface foam can be controlled with known defoamers. These consist, for example, of polyorganosiloxanes as described in U.S. Pat. No. 3,235,509 A, of polyorganosiloxanes in combination with polyoxyalkylenes as described in U.S. Pat. No. 3,984,347 A, or else of polyoxyalkylenes alone as described in “Antifoaming action of polyoxyethylene-polyoxypropylene-polyoxyethylene-type triblock copolymers on BSA foams”, Nemeth, Zs.; Racz, Gy.; Koczo, K. Colloids Surf., A, 127(1-3), 151-162, 1997.
DE 1444442 A1 discloses that foam destruction in hydraulic fluids containing approx. 50% glycols and polyglycols can be improved by the chemically related polypropylene glycol.
Conventional defoamers are known to be suitable for the control of “dry” surface foam, in which large gas bubbles are separated by thin liquid films (as described in Langmuir 2004, 20, 9463-9505). However, they are ineffective for deaeration of liquid-gas mixtures consisting mainly of liquid, with or without suspended solids.
This is because the surface properties and the solubility of defoamers which destroy the surface foam, which is also referred to as “macrofoam”, necessarily differ from the properties of deaerators (see Adams, J. W. et al. Verfkroniek, 68 (10) 1996 p. 43-45). Defoamers must be incompatible and migrate rapidly to the surface. Deaerators which, in contrast, are supposed to control the microfoam must have better compatibility since they are supposed to be effective not at the surface but in the liquid phase. It is therefore impossible to infer deaerator efficacy from a good defoamer efficacy (cf. EP 257 356 B1, page 2, lines 28-31).
Therefore, specific formulations are proposed for these applications. GB 2 350 117 A proposes, for better deaeration, use of linear or cyclic siloxanes bearing Si—C— or Si—O—C-bonded polyether groups. EP 257 356 B1 claims siloxanes with (isobutyryloxy)isopropyldimethyl-propoxy groups, which are said to enable better deaeration of plastisols than polyethersiloxanes.
There is still a need for better and more economic deaerating agents for various applications, especially for the production of pulp.